This invention relates in general to methods of printing images in color on sheet surfaces and, more particularly, to a method of printing very high quality color images on textured surfaces, such as canvas.
A very great number of methods have been developed for rapidly printing a large number of multi-color images on sheet material, generally paper or the like. In the most common color printing method, an original multi-color image is photographed through halftone screens with color filters to reproduce each of the three subtractive primary colors, cyan, yellow and magenta, plus black. Conventional halftone screens have a regular grid pattern formed by intersecting opaque lines on a clear substrate, such as glass or plastic, leaving an array of clear dots. The screens break the photographic image into evenly spaced dots that are larger in size in the dark areas of the image and smaller in light areas. The greater the number of dots per inch, the greater the image's sharpness and detail. There is a limit to how fine the screens can be made without printing problems. Also, if the screens are not correctly oriented, typically at 70.degree. to each other, Moire patterns may result, producing a distracting wavy pattern across the image. While halftone printing with modern materials gives excellent color rendition when properly done, these problems limit ultimate quality obtainable.
Recently, a method of printing has been developed that uses random dots instead of the regular dot pattern of conventional halftone. This method is described, for example, by Rapoport et al. in U.S. Pat. No. 4,037,533. Through computer techniques, the distribution of very small dots is carefully randomized and the dots are placed more or less often as required to reproduce the image, in no determinable pattern. In light colored areas, single spaced dots will appear, while in heavily colored areas the dots form in randomly-shaped clumps. This method is generally called "stochastic" or "FM" screening. This substantially eliminates Moire patterns and "rosette" color clumps while simplifying printing with more than four colors. However, there are several problems with using stochastic screening with most conventional overall printing processes. Many of these processes are not sensitive enough to capture all of the available detail, especially in proofing.
Since the earliest days of lithography, printing plates have been fastened around a cylinder and dampened with a fountain solution, a mildly acidic or neutral liquid that usually consists of water and chemical additives. The water repels the inks, which are water-resistant, from the non-image areas of the printing plate so that when the plate is inked, the ink adheres only to the areas that contain an image. The plate is then pressed against a rubber blanket to which the image is transferred. The image is then retransferred onto paper or other surfaces to transfer the ink image. With fine dot pattern plates, the milling effect of the press rollers can cause the ink to become emulsified to the point where the fine pattern is not reproduced. Special silicone coated plates have been developed that do not require a fountain solution. A typical waterless printing process is described by Abiko et al. in U.S. Pat. No. 4,259,905. Much more detailed dot patterns can be printed. However, there are problems in printing by this waterless technique. Precise temperature control of the ink is required, which may vary with inks of different colors. The printing plates may undergo excessive wear.
With either the water based or waterless printing system, the ink applied in the first step of multi-color printing will still be wet when the next color is applied. This can result in latent image transfer, which occurs when a wet ink laid down by one print unit is transferred to the next unit and subsequently laid down again along with the color image being printed by the current unit. This produces poor quality, blurred, final prints. In some cases an anti-offset powder is applied to the print medium between printing steps. While helpful in drying the ink, the powder tends to produce a rough surface on the printing medium, such as paper, when it becomes embedded in the material. Also, the ink must be allowed to fully dry before the second side of the material can be printed. Ideally, the ink would be dried after one color is applied to the printing medium before the next color can be applied. However, this slows the printing process excessively.
Inks that can be dried by exposure to ultraviolet light (UV) or electron beams between printing steps have been developed. Printing using these inks is generally referred to as "dry trap" printing. Typically, ultraviolet driers are positioned between succeeding print units. Because the inks are dried immediately, they are not absorbed into the paper, giving higher density images for a given quantity of ink and lower grades of paper may be used without excessive bleeding.
While dry trap printing gives excellent image quality at high speed, the inks are relatively expensive and the series of UV driers must be provided. The inks dry dull, without any gloss, so that overcoating with a gloss coating is necessary for glossy images.
While each of the stochastic, waterless and dry trap processes, when carefully used, will produce excellent printing detail, they generally require high quality, smooth, paper surfaces for optimum print quality. A smooth surface texture results.
For high quality art prints it is often desirable to print on canvas or other textured surfaces. Further, enhancing an art image with variable surface texture effects, translucent overcoated colors, etc. often is desirable to enhance artistic effects.
Thus, there is a continuing need for improved printing processes to provide very high quality, highly detailed images on canvas or textured surfaces.